Not Applicable.
Not Applicable.
The present invention generally relates to a method for making ceramic-embedded micro-electromagnetic devices such as ceramic-embedded micro-antennas, and the devices made therewith. The present invention is further directed to making a ceramic-embedded helical micro-antenna which is particularly advantageous for use in the upper MHz and THz frequency range.
The current wireless revolution is spawning a plethora of new wireless communication and data processing devices making information and voice data instantly available virtually anywhere in the world.
A common feature of such devices is the need for reduced physical size and increased functionality. For example, there is a growing trend to incorporate GPS (Global Positioning Systems) and Bluetooth (TM) technology in consumer electronics devices such as personal digital assistants (PDAs), notebook computers, digital cameras and wireless phones. Bluetooth (TM) is a specification for a small form-factor, low-cost, short-range, cable-replacement radio technology used to link notebook computers, mobile phones and other portable handheld devices, as well as for connectivity to the Internet.
The large number of passives needed for filtering and impedance matching elements associated with these technologies can quickly add up to a significant amount of space and integrating them either on the main printed circuit board (PCB) or on the substrate at a module level can realize important cost and size advantages.
A particularly difficult function to integrate is the antenna. Bluetooth (TM) designers have identified embedded antennas as the most viable alternative. Of all compact antenna configurations, the ceramic embedded helical antenna offers the greatest potential for small size with respectable gain. Embedded antennas are also a rugged and durable solution for compact mobile phones, providing exceptional clarity and being suitable for multi-band reception. They can be unobtrusively hidden within the handset.
Another important issue is the effect of antenna design on SAR (Specific Absorption Rate) levels. Measurements suggest that 40% of the RF power from a mobile phone in either the 800-MHz or 1900-MHz band is absorbed by the user""s head when an omni-directional antenna is used. Hence, antennas must be designed so that field emissions in the direction of the user will be below the regulatory limits for maximum SAR. Ceramic embedded antennas can be installed very close to electronic circuits, mechanical objects and human tissue. Their near field is enclosed within the ceramic core of the antenna. This antenna technology also reduces the need for filters and for a large ground plane, thereby lowering component costs and handset interaction. Another notable advantage for handheld mobile telephones is that the ceramic core largely voids detuning when the antenna is brought close to the head of the user.
Portable communicators, such as cell phones, frequently utilize helical or helix antennas. Helical windings permit a relatively long effective antenna length by reducing the helical pitch. This is convenient in cell phones and other portable communicators since small physical size is beneficial and since a certain antenna length is necessary to achieve particular broadcast and reception frequencies.
Helical antennas are usually formed from a thin and delicate conductive wire. Thin wires help preserve the small size and low weight desirable in portable communicators while facilitating low power transmission and reception. This requires the helical conductor to be encased in a protective material, since cell phone antennas are often subjected to forces, which could permanently deform the delicate helical windings.
Based upon the radio frequency response requirements of each individual application, the dimensions of the wire diameter, overall length, outside coil diameter, pitch angle, etc. can be altered.
Helical antennas typically comprise a coil wound around a central core. The process of winding the core is a complicated and expensive process, generally requiring production and assembly of multiple parts and precision winding of a fine wire.
Where circular polarization is desired, the helical antenna has been typically configured as a multi-winding structure comprised of a plurality of concentrically arranged helical windings, each having a fractional number of turns, and terminating the respective windings to a multi-quadrature port hybrid interface.
However, as operational frequencies have reached into the multidigit GHz range, achieving dimensional tolerances in large numbers of identical components has become a major challenge to system designers and manufacturers. For example, in a relatively large number element phased array antenna operating at frequency in a range of 15-35 GHz, and containing several hundred to a thousand or more antenna elements, each antenna element may have on the order of twenty turns helically wound within a length of only several inches and a diameter of less than a quarter of an inch.
While conventional fabrication techniques may be sufficient to form helical windings for relatively large sized applications, they are inadequate for very small sized (multi-GHz applications) where minute parametric variations are reflected as substantial percentage of the dimensions of each element. As a consequence, unless each element is identically configured to conform with a given specification, there is no assurance that the antenna will perform as intended. This lack of predictability is often fatal to the successful manufacture and deployment of a high numbered multi-element antenna structure, especially one that may have up to a thousand elements.
An impressive number of recent inventions cover the design of helical antennas. Simple helical antenna designs are disclosed in Saito, U.S. Pat. No. 6,097,341; Fahlberg, U.S. Pat. No. 6,107,966; Tassoudji et al., U.S. Pat. No. 6,107,977; Chenoweth et al. and U.S. Pat. No. 6,166,696.
Nevermann et al., U.S. Patent Application Publication No. 2001/0005183 and Richter et al., PCT Patent No. WO 01/56111, all describe helical structures composed of strip-shaped flat antenna elements while Filipovic, U.S. Pat. No. 6,278,414 discloses a bent-segment helical antenna.
A dual helical switchable antenna system is taught by Lee et al., U.S. Pat. No. 6,249,262, while Barts et al., U.S. Pat. No. 5,986,621 attempt to reduce the physical outer dimensions of helical antennas by incorporating several incremental folds in the conductor. A dual pitch helical antenna is the subject of Volman, U.S. Pat. No. 6,172,655.
Bengtsson et al., U.S. Pat. No. 6,259,420 describe an antenna system with four interwoven helical wires while Van Voorhies, U.S. Pat. No. 6.239,760 discloses a counterwound toroidal helical antenna.
In the field of cardiac surgery, Moss et al., U.S. Pat. No. 5,741,249, disclose a microwave ablation catheter incorporating a helical antenna coil adapted to radiate electromagnetic energy in the microwave frequency range. The antenna coil typically has a diameter of about 1.7-2.5 mm. Another catheter system for ablation of body tissues, also incorporating a helical antenna, is disclosed in Ormsby et al., U.S. Pat. No. 6,190,382.
Goldstein, U.S. Pat. No. 6,166,709, attempts to improve on monofilar antenna design in order to obviate the complexities of manufacture of multifilar antennas. Multifilar antennas, used primarily as satellite antennas, require several radiating elements running parallel to each other while spiralling around a common center axis. Bifilar, quadrifilar, hexafilar and multifilar antenna designs are in use. It is very important for the different conductive elements to be held in a precise location with respect to each other both radially and axially. Hence, multifilar antennas are difficult to manufacture at the required tolerance.
Sanford, U.S. Pat. No. 6,094,178; Winter et al., U.S. Pat. No. 6,150,994; Teran, U.S. Pat. No. 6,160,516; Ho, U.S. Pat. No. 6,160,523 and Kiesi, U.S. Pat. No. 6,212,413 all disclose quadrifilar antenna designs while Ho, U.S. Pat. No. 6,157,346 and Matsuyoshi, U.S. Pat. No. 6,278,415 teach a hexafilar and multifilar antenna design respectively.
The problems encountered in multifilar antenna fabrication are exemplified in Sullivan, U.S. Pat. No. 6,137,452 who discloses a multifilar antenna design in which helical grooves on the outer and optionally inner surface of a cylinder made from a non-platable plastic are filled with a platable plastic. The exposed surface of the filled grooves is then plated to form a helical conductor. When the platable plastic is injected into the grooves any surfaces that are not to be coated or filled must be blanked off by the mold cavity walls or cores. Hence the need for high injection velocity and pressure.
For reasons of physical and electrical stability, the material of the antenna core is preferably a microwave ceramic material with a high relative dielectric constant such PZT (lead zirconium titanate), magnesium calcium titanate, barium zirconium tantalate, barium neodymium titanate, or a combination of these. Such materials have negligible dielectric loss to the extent that the Q of the antenna is governed more by the electrical resistance of the antenna than core loss. The actual frequency of resonance of the resonator depends on the relative dielectric constant of the ceramic material forming the core.
With a core material having a relative dielectric strength of about 36, an antenna designed for L-band GPS reception at 1575 MHz typically has a core diameter of about 5 mm and the longitudinally extending antenna elements a longitudinal extent, parallel to the central axis, of about 8 mm. As a result of the very small dimensions of these antennas, manufacturing tolerances may be such that the precision with which the resonant frequency of the antenna can be maintained is insufficient. A significant source of variation in resonant frequency is the variability of the relative dielectric constant of the core material. This usually requires test samples to be produced from each new batch of ceramic.
Zhou et al., U.S. Pat. No. 6,127,979 describe a helical coil antenna fitted with a plastic dielectric core and then insert molded, while Gasparaitis et al., U.S. Pat. No. 4,725,395, teach a helical coil antenna embedded in plastic via a double insert molding operation.
Bumsted, U.S. Pat. No. 5,648,788, recognizing the need for high injection pressures and high injection speeds and the inherent potential for deformation of the coil spring during insert molding, discloses a relatively complex tool assembly on which several coils are positioned. The loaded tool is then manually placed inside the mold, thereby blocking the coils in place during insert molding.
Chufarovsky et al., U.S. Pat. No. 6,111,554 disclose a coil spring first screwed over a plastic core and then insert molded.
Zandbergen, U.S. Pat. No. 4,435,716 teaches a plastic embedded helical antenna by tightly winding a somewhat resilient but deformable conductor wire, typically aluminum wire of 1.6 mm diameter, over a tapered mandrel, removing the wound coil from the mandrel and pulling it through the inner periphery of a hollow frustoconical plastic antenna casing so as to give the coil the desired length and pitch, following which the remaining void inner space is filled with an epoxy.
Valimaa et al., U.S. Pat. No. 5,341,149, also recognizing the potential for thin helical windings to deform during insert molding, disclose a grooved core, around which the helical coil is first wound prior to insert molding the core-coil assembly.
Kulisan et al., U.S. Pat. No. 6,181,296 machine a helical groove in a mandrel. A wire is placed inside the groove and silicone cast around the wound mandrel. After curing of the silicone the mandrel is extracted and a dielectric glass bead-epoxy mixture cast into the silicone mold. After curing, the casting is removed from the silicone mold and used as a dielectric core around which the antenna wire is wound.
Memmen et al., U.S. Pat. No. 6,219,902 disclose a threaded bolt on which a coil spring is screwed to support the latter during insert molding. After molding, the bolt is removed and the space left behind optionally filled with a dielectric core or with plastic.
Lin et al., U.S. Pat. No. 6,229,488 describe a combined helical and patch antenna with a ceramic core, while Leisten et al., U.S. Pat. No. 6,184,845, and Leisten, U.S. Pat. No. 6,181,297, disclose a bifilar and quadrifilar helical antenna with ceramic core respectively.
Elliott, U.S. Pat. No. 6,147,660 attempts to obviate the wire winding step by forming the helical antenna shape directly via the metal injection molding (MIM) process. However, the skilled in the art will instantly realize that this is not so simple. Indeed, regardless of the materials molded, i.e. metals, metal-filled plastics or unfilled plastics, there is obviously a first requirement to provide a mold with a mold cavity insert in the shape of the desired helical coil. Such mold inserts would be extremely difficult and very costly to fabricate, and the more so the smaller the dimensions of the end product.
Furthermore, as is again well known to those skilled in the art, molding a helical path is in itself very difficult, particularly as product dimensions shrink. This is mainly due to the rapid pressure drop in cavities with high aspect ratios such as capillary channels, whether helical in shape or not. The classical spiral mold test used in the plastics industry to evaluate the flow properties of plastic materials is precisely based on the principle of high pressure drop to stop the flow inside the spiral channel. Hence, the filling of a helical mold cavity rapidly becomes impractical or impossible due to the need to apply unusually high injection pressures and temperatures. For the same reasons the ejection of parts molded in helical mold cavities poses serious technical and practical problems.
It will also be obvious to those skilled in the art of metal injection molding, that maintaining shape integrity during sintering of a binder-free green helical coil would pose enormous challenges due to the inherent shrinkage upon sintering, usually in the range of 15-25% linear or about 40-60% by volume. This problem is further exacerbated by the fact that the organic binder in metal injection molded parts must be totally removed from the green parts prior to the onset of sintering. At that moment the residual tensile strength of the green parts is too weak to resist the pull of the earth""s gravitational field, resulting in distortion. Only sintering in the low gravity environment of outer space would obviate this problem.
An area of great interest and potential is the THz region of the electromagnetic spectrum with many applications in the medical field, for example, in MRI (Magnetic Resonance Imaging). The current art uses planar microstrip antennas, which do not provide a true 3-D structure needed for performance under certain conditions, e.g. circular polarization in the THz frequency range. Fabrication of helical antennas for this frequency range poses serious technological challenges as dimensions become so small. As an example, typical approximate major dimensions of a helical antenna operating at 1 THz would be:
Clark et al., U.S. Pat. No. 6,271,802 describe a method to grow a helical micro-antenna on the surface of a silicon substrate by LCVD (Laser Chemical Vapor Deposition) technology.
In conclusion, as can be inferred from the above review of the prior art, antenna manufacture for advanced wireless applications is strewn with major technological hurdles.
A low-cost method for fabricating ceramic embedded helical antennas and particularly antennas designed to operate in the GHz and THz frequency range would greatly benefit the development of advanced wireless technology.
Furthermore, many other applications requiring small and precisely formed electromagnetic coils would also benefit from such a low cost manufacturing method.
In accordance with the present invention an economic and environmentally benign method is provided to fabricate ceramic-embedded micro-electromagnetic devices by first producing ceramic bodies containing complex capillary helical channels which are subsequently filled with metal.
It is a primary object of this invention to provide a micro-electromagnetic device consisting of a ceramic housing incorporating complex internal metal-filled channels.
It is another object of this invention to provide a method to fabricate micro-electromagnetic devices.
Yet another object of the present invention is to provide ceramic-embedded micro-antennas.
Still another object of the present invention is to provide a method to fabricate ceramic-embedded micro-antennas.
The invention allows the fabrication of arrays of ceramic embedded micro-electromagnetic devices as well as ceramic embedded helical micro-antennas design for use in the high GHz and THz regions at a fraction of the present cost of manufacturing of such devices and with virtually no restriction to their miniaturization.